The present invention relates to the control of a combustion process in a boiler, heater, or other device in which fuel and air are combined and burned to produce heat.
Techniques are known in the area of combustion control which involve the measurement of various products of combustion in the flue gases and the use of these measurements to adjust the amount of excess air (or air/fuel ratio) supplied beyond the stoichiometric level required for ideal combustion. The prior art recognizes that there is a tradeoff between a high level of excess air, in which air heating losses predominate, and too low a level of excess air, in which unburned fuel losses predominate.
Prior approaches to optimizing the combustion process fall into one of three categories, depending on what product or products of combustion are being measured in the flue gases: oxygen only, combustibles only, or a combination of the two. These are discussed separately in the following.
The oxygen only approach is used in the Bailey Meter Company U.S. Pat. No. 3,049,300, "Combustion Control for a Furnace Fired With Fuels Having Different Oxygen-Excess Air Characteristics," dated Aug. 14, 1962. An analyzer is used to measure the oxygen in the flue gas, and the excess air is reduced until the measured oxygen reaches a preselected set point.
The combustibles only (Carbon monoxide-CO, hydrocarbons, and/or opacity) approach is used in Standard Oil Company (Indiana) U.S. Pat. No. 4,260,363, "Furnace Fuel Optimizer," dated Apr. 7, 1981, and the copending application to the Econics Corporation, referenced in a technical paper by Keith Swanson, "An Advanced Combustion Control System Using Distributed Microcomputer Techniques," ISA Publication ISBN 0-87664-521-X, 1981. An analyzer or analyzers are used to measure one or more of these parameters, and excess air is adjusted until they reach a preselected set point. If more than one variable is measured and controlled, some switching between controlled variables is done to attain the most "conservative" value of excess air.
The combination of oxygen and combustibles approach is used in the Measurex Corporation U.S. Pat. No. 4,162,889, "Method and Apparatus for Control of Efficiency of Combustion in a Furnace," dated July 31, 1979, and Westinghouse Electric Corporation U.S. Pat. No. 4,231,733, "Combined O2/Combustibles Solid Electrolyte Gas Monitoring Device," dated Nov. 4, 1980 and a copending application to the Bailey Controls Company, "A System for CO and O2 Control of Combustion Processes." In this case, both oxygen and combustibles are measured. In Measurex patent and the copending application, the deviation of CO from its preselected set point is used to adjust the set point of an O2 controller in a cascade fashion. In the Westinghouse Patent, excess air is adjusted to control, to a preselected combustibles set point, until the oxygen moves outside preselected limits. Then the control mode is switched to bring the oxygen back within limits, at which point combustibles control is resumed.
The shortcomings of the current aproaches to combustion control are as follows:
All of the approaches attempt to control to arbitrary selected set points one or more of the products of combustion. There is no guarantee that combustion conditions are such that these set points can be reached or that these set points are the best ones from an economic point of view however.
In approaches that attempt to switch among multiple variables to be controlled, it is likely that limit cycling will occur as the various switch points are reached and the modes of control change. This leads to undesirable cyclic stresses on the process equipment.
None of the approaches attempts to directly minimize any explicit measure of economic loss, such as the cost of unburned fuel up the stack, the cost of heating the excess air, or the cost of violating government emission regulations.